The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appear to be promising. Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to Michel Borel et al.; U.S. Pat. No. 5, 194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
A FED flat panel display arrangement is disclosed in U.S. Pat. No. 4,857,799, "Matrix-Addressed Flat Panel Display," issued Aug. 15, 1989, to Charles A. Spindt et al., incorporated herein by reference. This arrangement includes a matrix array of individually addressable light generating means of the cathodoluminescent type having electron emitting cathodes combined with an anode which is a luminescing means of the CRT type which reacts to electron bombardment by emitting visible light. Each cathode is itself an array of thin film field emission cathodes on a backing plate, and the luminescing means is provided as a phosphor coating on a transparent face plate which is closely spaced to the cathodes.
The emitter backing plate disclosed in the Spindt et al. ('799) patent includes a large number of vertical conductive cathode electrodes which are mutually parallel and extend across the backing plate and are individually addressable. Each backing plate includes a multiplicity of spaced-apart electron emitting tips which project upwardly from the vertical cathode electrodes on the backing plate and therefore extend perpendicularly away from the backing plate. An electrically conductive gate electrode arrangement is positioned adjacent to the tips to generate and control the electron emission. The gate electrode arrangement comprises a large number of individually addressable, horizontal electrode stripes which are mutually parallel and extend along the backing plate orthogonal to the cathode electrodes, and which include apertures through which emitted electrons may pass. Each gate electrode is common to a full row of pixels extending across the front face of the backing plate and is electrically isolated from the arrangement of cathode electrodes. The emitter back plate and the anode face plate are parallel and spaced apart.
The anode is a thin film of an electrically conductive transparent material, such as indium tin oxide, which covers the interior surface of the face plate. Deposited onto this metal layer is a luminescent material, such as phosphor, that emits light when bombarded by electrons.
The array of emitting tips are activated by addressing the orthogonally related cathode gate electrodes in a generally conventional matrix-addressing scheme. The appropriate cathode electrodes of the display along a selected stripe, such as along one column, are energized while the remaining cathode electrodes are not energized. Gate electrodes of a selected stripe orthogonal to the selected cathode electrode are also energized while the remaining gate electrodes are not energized, with the result that the emitting tips of a pixel at the intersection of the selected cathode and gate electrodes will be simultaneously energized, emitting electrons so as to provide the desired pixel display.
The Spindt et al. patent teaches that it is preferable that an entire row of pixels be simultaneously energized, rather than energization of individual pixels. According to this scheme, sequential lines are energized to provide a display frame, as opposed to sequential energization of individual pixels in a raster scan manner.
The Clerc ('820) patent discloses a trichromatic field emission flat panel display having a first substrate comprising the cathode and gate electrodes, and having a second substrate facing the first, including regularly spaced, parallel conductive stripes comprising the anode electrode. These stripes are alternately covered by a first material luminescing in the red, a second material luminescing in the green, and a third material luminescing in the blue, the conductive stripes covered by the same luminescent material being electrically interconnected.
Today, a conventional FED is manufactured by combining the teachings of many practitioners, including the teachings of the Spindt et al. ('799) and Clerc ('820) patents. Referring initially to FIG. 1, there is shown, in cross-sectional view, a portion of an illustrative field emission device in which the present invention may be incorporated. In this embodiment, the field emission device comprises an anode plate 1 having an electroluminescent phosphor coating 3 facing an emitter plate 2, the phosphor coating 3 being observed from the side opposite to its excitation.
More specifically, the field emission device of FIG. 1 comprises a cathodoluminescent anode plate 1 and an electron emitter (or cathode) plate 2. A cathode portion of emitter plate 2 includes conductors 9 formed on an insulating substrate 10, an electrically resistive layer 8 which is formed on substrate 10 and overlaying the conductors 9, and a multiplicity of electrically conductive microtips 5 formed on the resistive layer 8. In this example, the conductors 9 comprise a mesh structure, and microtip emitters 5 are configured as a matrix within the mesh spacings. Microtips 5 take the shape of cones which are formed within apertures through conductive layer 6 and insulating layer 7.
A gate electrode comprises the layer of the electrically conductive material 6 which is deposited on the insulating layer 7. The thicknesses of gate electrode layer 6 and insulating layer 7 are chosen in such a way that the apex of each microtip 5 is substantially level with the electrically conductive gate electrode layer 6. Conductive layer 6 may be in the form of a continuous layer across the surface of substrate 10; alternatively, it may comprise conductive bands across the surface of substrate 10.
Anode plate 1 comprises a transparent, electrically conductive film 12 deposited on a transparent planar support 13, such as glass, which is positioned facing gate electrode 6 and parallel thereto, the conductive film 12 being deposited on the surface of the glass support 13 directly facing gate electrode 6. Conductive film 12 may be in the form of a continuous layer across the surface of the glass support 13; alternatively, it may be in the form of electrically isolated stripes comprising three series of parallel conductive bands across the surface of the glass support 13, as shown in FIG. 1 and as taught in U.S. Pat. No. 5,225,820, to Clerc. By way of example, a suitable material for use as conductive film 12 may be indium-tin-oxide (ITO), which is optically transparent and electrically conductive. Anode plate 1 also comprises a cathodoluminescent phosphor coating 3, deposited over conductive film 12 so as to be directly facing and immediately adjacent gate electrode 6. In the Clerc patent, the conductive bands of each series are covered with a particulate phosphor coating which luminesces in one of the three primary colors, red, blue and green 3.sub.R, 3.sub.B, 3.sub.G.
Selected groupings of microtip emitters 5 of the above-described structure are energized by applying a negative potential to cathode electrode 9 relative to the gate electrode 6, via voltage supply 19, thereby inducing an electric field which draws electrons from the apexes of microtips 5. The potential between cathode electrode 9 and gate electrode 6 is approximately 70-100 volts. The freed electrons are accelerated toward the anode plate 1 which is positively biased by the application of a substantially larger positive voltage from voltage supply 11 coupled between the cathode electrode 9 and conductive film 12 functioning as the anode electrode. The potential between cathode electrode 9 and anode electrode 12 is approximately 300-800 volts. Energy from the electrons attracted to the anode conductive film 12 is transferred to particles of the phosphor coating 3, resulting in luminescence. The electron charge is transferred from phosphor coating 3 to conductive film 12, completing the electrical circuit to voltage supply 11. The image created by the phosphor stripes is observed from the anode side which is opposite to the phosphor excitation, as indicated in FIG. 1.
It is to be noted and understood that true scaling information is not intended to be conveyed by the relative sizes and positioning of the elements of anode plate 1 and the elements of emitter plate 2 as depicted in FIG. 1. For example, in a typical FED shown in FIG. 1 there are approximately one hundred arrays 4, of microtips and there are three color stripes 3.sub.R, 3.sub.B, 3.sub.G per display pixel.
The process of producing each frame of a display using a typical trichromatic field emission display includes a) applying an accelerating potential to the red anode stripes while sequentially addressing the gate electrodes (row lines) with the corresponding red video data for that frame applied to the cathode electrodes (column lines); b) switching the accelerating potential to the green anode stripes while sequentially addressing the rows lines for a second time with the corresponding green video data for that frame applied to the column lines; and c) switching the accelerating potential to the blue anode stripes while sequentially addressing the row lines for a third time with the corresponding blue video data for that frame applied to the column lines. This process is repeated for each display frame.
Pen-based computer systems are computer systems which use an electronic pen or stylus, instead of, or in addition to, a keyboard, to enter data and to control various computer functions, by writing, sketching, and pointing, directly on the computer display. The computer systems which utilize pen-based technology may be portable, e.g. battery-operated and less than four pounds; they may be desktop, e.g. AC-powered and transportable but not designed to operate while being moved; and they may be terminal-based, e.g. used as a terminal in communicating with a mainframe computer on a network or via a modem.
The portable market segment is targeting forms entry applications for inventory, insurance, delivery vehicles, field service, nursing/healthcare, law enforcement, and industries where workers are accustomed to filing in forms on a clipboard. Desktop systems are targeted at executives, stock brokers, business, and office automation. The terminal based market includes factory automation, industrial control, and point-of-sale.
There are basically four pen-input technologies used in pen-based computers: 1) a direct contact, resistive technology with indium-tin oxide deposited on the top surface of the anode glass support 13 of the field emission device display, 2) a wire grid construction implemented in indium-tin oxide coating which is located on a multi-layer glass assembly placed on top of the anode, 3) a conventional wire-grid electromagnetic digitizer placed underneath the field emission device display, and 4) a position responsive surface or digitizer having writing surfaces formed of a continuous resistive material located on a supportive substrate such as glass or plastic placed on top of the display. The stylus used in these systems may be corded or cordless.
The first technology, direct-contact resistive, is an adaptation of an older touch screen technology and requires that the indium-tin oxide coating be placed on the top surface of the FED anode glass support 13 so that the stylus can directly contact the electrically active layer, a so-called "direct-contact" technology. It is well known that this method of construction is subject to scratches and wear during normal operation and will not meet the durability requirements of the computer products industry. A further drawback of the direct contact digitizer is that the system does not respond to the user until the stylus physically touches the screen, making cursor control more difficult for the user.
The second technology, while having good performance and durability, is considerably more complex and costly to implement. This is because the wire grid structure requires a minimum of two sheets of coated glass, the etching of high-tolerance "wires" in the indium-tin oxide coating, and a complex system of custom driver circuits attached to each row and column of the sensor grid.
The third technology, wire-grid electromagnetic, has implementation problems associated with the fact that the digitizer is located approximately one-half inch below the writing surface. Tooling costs are generally higher than that of other technologies. Each wire in the sensor grid (typically between 50 and 200 individual wires, or more for higher resolution systems) must be connected to an electronic controller circuit. Electromagnetic digitizers also typically require a plane of magnetic material behind the wire-grid sensor to shield the system from stray magnetic effects. The additional weight of the wire-grid digitizer underneath the display adds unnecessary weight to the unit.
The fourth technology, resistive layer type electrographic digitizers, also has many disadvantages. First, there are problems with erroneous position readings caused by stray capacitance. Since computer screens are rectangular and not square, the edge-to-edge resistances in the horizontal direction is not the same as in the vertical direction; therefore, the resolution of the system is not the same in both directions. Furthermore, resistive layer electrographic digitizers are sensitive to electrical interference from externally generated noise and hand effects.
FIG. 2 shows an example of how current pen-based technology is implemented in FED systems using the fourth method. The digitizer consists of a single substrate of soda-lime float glass 17 coated on the underneath surface with a single layer of indium-tin-oxide (ITO) 15. Plastic is sometimes laminated to the glass panel 17 for added strength. Digitizer circuitry 14 is mounted to the face of the flat panel display. Electrical connection to the ITO is made by a silver wiring pattern on the periphery of the glass. The controller 16 signals the digitizer circuitry 14 to generate a 100 kHz AC drive voltage. Analog switches distribute drive signals to the four corners of the resistive ITO layer, creating an AC voltage gradient which alternates between the x and y axis. The tip of stylus 20, located on the glass surface opposite the ITO, detects the local magnitude of the applied voltage gradient by AC capacitive coupling. Analog circuits (not shown) amplify and filter the AC position signals and convert them to digital coordinate data. The controller then sends the appropriate display data to the display driver 18 of the flat panel display 14.
All four of the pen-input technologies described above have resolution granularity problems, the detection grid and the video display more often than not do not map precisely to the same x,y location. For this reason, pen-input data that the digitizer reports must be calibrated to approximate the location of the video pixels viewed by the user. This calibration often makes correct recognition more difficult than it would be if the digitizer and the video display coordinates were precisely the same.
Another problem with all four of the pen-input technologies described above is that additional components must be added to the pen-based systems. Components such as a controller, converters, numerous switching circuits, and other supportive devices must be added to the system electronics. These additional components increase the complexity of the system, reduce system reliability, and increase system cost.
What is needed is an pen-based system that is integrated into and utilizes existing FED circuitry. More ideally, what is needed is a pen-based system which doesn't require calibration between the detection circuitry and the display pixel circuitry. Also what is needed is a pen-based system which is lower in cost and which consumes less power than current pen stylus technologies.